This invention relates to microtomes, more particularly ultramicrotomes, of the type having a specimen carrier which can be moved relative to a knife, the specimen carrier possessing a clamping aperture for the purpose of holding a specimen, and a light source for illuminating the specimen.
The majority of specimens cannot be sliced in their natural form and condition. In most cases, the preparation of such specimens (small pieces of animal or human organs, such as liver, spleen or brain, residues centrifuged from body fluids, parts of plants or microorganisms, or tissue-cultures, etc.) is required. This usually involves a stabilization treatment by means of an aldehyde and/or a heavy metal, followed by dewatering and embedding the specimen in a plastic material which is normally transparent or translucent (e.g. an epoxide or a polyester).
The cylindrical, flat or prismatic, "specimen blocks" which are obtained by means of this process, and which can readily be cut, are clamped in a specimen carrier and, after trimming to size, the specimen blocks are sliced into the thin sections for examination under a microscope or an electron microscope. The slicing operation is usually carried out with a knife made of metal, glass or diamond.
The ability to observe the specimen/knife region in an extremely precise manner is a prerequisite for the production of perfect thin sections in a microtome and especially in an ultramicrotome. In particular, the success of a slicing operation of this type depends critically on selecting the correct region of the specimen block. For this purpose, especially in the case of ultramicrotomes, a stereomicroscope and a light source are provided, the former for observing the specimen/knife region and the latter for illuminating it. The light source is installed above the specimen/knife region, and/or beneath it, and is frequently adjustable in order to enable it to be aligned as precisely as possible with the cutting edge of the knife, and/or the surface of the specimen block to be cut.
Up to the present time, the means for enabling the specimen/knife region to be observed in a precise manner has not been completely satisfactory. Thus, it is not always possible for an operator to select the correct region of the specimen block because the internal structure of the specimens, the fine surface structure, or very small objects, are poorly displayed and cannot be identified so reliably that the details of interest occur in the selected slice. Sometimes interesting regions of the specimen are unintentionally removed during the trimming operation, and have hence been lost, so that the surface of the prepared section contains regions which not only are of no interest, but which may interfere with the cutting process (e.g. hard inclusions, collagen fibers which are difficult to cut, or fat cells). Neither incident-light illumination by means of cold-light tubes or similar light sources above the specimen, as generally employed, nor dark-field illumination, which is often employed to illuminate the specimen/knife region from below (sublevel illumination), is satisfactory to facilitate identifying the desired areas of the specimen. The only effective procedure involves trimming the specimen while it is fastened to the specimen carrier, during which process it is observed from the front by means of a mirror system ("structure viewer").
Furthermore, when an adjustment of the light source (indicent-light illumination, and/or sublevel illumination), or a setting of the specimen in relation to a light source, has been found to give optimum illumination of the specimen, it is exceedingly irritating if the position of the specimen, relative to the knife, has to be altered in order to obtain thin sections of optimum quality. This is because the illumination must be readjusted in order to ensure that the desired precision of observation is maintained. This situation arises, for example, in the case of specimen carriers which are combined, for the purpose of angular adjustment, with so-called segment arcs, along which they can be swung horizontally about a pivot axis which lies approximately on the cutting edge of the knife.